For some years now, three-dimensional (3-D) x-ray inspection systems have been a popular alternative to previously available physical inspection and diagnosis technologies: Such systems are now commonly used for defect analysis and quality inspection of manufactured articles, such as electronic printed circuit boards (PCBs). The use of these systems allows rather detailed inspection of areas of an article that are either too small to be seen with the naked eye, or are obscured from direct view.
Several types of 3-D x-ray inspection systems are now available, each with their own inherent advantages and limitations. For example, x-ray laminography systems, such as the one described in U.S. Pat. No. 4,926,452 by Baker et al., utilize an angled, rotating x-ray source in conjunction with a moving area image detector to acquire an image of a single planar layer of an article under inspection. Due to the rotational movement of the source and the image area detector, the layer of concern within an area under inspection is viewed from a continuous range of oblique viewpoints so that other layers of the article do not remain stationary within the area of view. This movement essentially causes those other layers to fade from the resulting image. The result is that only those features within the layer that reside in the “focal plane” described by the rotating x-ray source and detector are prominent.
While x-ray laminography systems are exceptionally useful in many applications, such systems require rather expensive and complex technology, including the x-ray tube and drive electronics needed to implement the precise movements of an electron beam within the tube used to generate the rotating source of x-rays. Also, the movement of the area image detector must be accurately coordinated with the motion of the x-ray source. Precise mechanics and electronics for moving the article under examination both horizontally and vertically are typically necessary so that both the area and the layer of the article to be inspected lie within the system focal plane. Furthermore, due to the rather small areas such systems are capable of inspecting at any one time, the number of areas and layers that are normally required to fully inspect an article are typically rather large. As a result, such a system may require a rather protracted amount of time to perform a complete examination of each article. Additionally, laminography systems normally require the execution of a preliminary process called “surface mapping” for each article to be inspected. This mapping essentially measures the height of numerous locations on the surface of the article under inspection so that proper positioning of the article within the system focal place for each small inspection area may be accomplished.
Another category of x-ray inspection systems similarly involves the use of a moving x-ray source. However, instead of generating a continuous moving image during a rotation of the source, two or more discrete images are generated by way of a single large stationary image intensifier or several smaller stationary area image sensors. Such systems, examples of which are described by Adams et al. in U.S. Pat. No. Re. 35,423 and by Peugeot in U.S. Pat. No. 5,020,086, allow the x-ray source to dwell at particular angles through the area of interest on the article. The resulting discrete image at each beam orientation is then stored digitally. All of the images for a particular area and layer of the article under inspection are then mathematically processed by way of either computer hardware or software so that a single image representing the area and layer under inspection may be generated. Such inspection systems eliminate the need for precise coordination of image sensor movement with that of the x-ray source. However, the moving image sensor is replaced by multiple, expensive x-ray area image sensors, or in the alternative by a large image intensifier that normally exhibits reduced resolution and increased geometric distortion at a possibly higher cost.
An alternate x-ray inspection system, as discussed in U.S. Pat. No. 5,583,904 issued to Adams, uses one or two x-ray tubes in conjunction with two to four linear x-ray image sensors. The x-ray sources of the tubes do not rotate, but require the use of collimators and shields to guide the x-rays appropriately onto the image sensors. The article to be inspected is then transported horizontally across the linear sensors, each of which must be long enough to allow an image across the entire width of the article in a single pass. This requirement thus results in either a limit on the size of articles to be inspected, or in higher costs resulting from the use of exceptionally long linear sensors. As the board passes by the sensors, each sensor acquires a series of sequential linear images which are subsequently stored for later computer processing to generate an image for each layer of the article. While such a system decreases the total amount of inspection time for a particular article by limiting the movement of the board to a single linear pass across the sensors, the number and variety of angles that can be implemented to capture quality images of the article layers are severely limited. Additionally, the use of two x-ray tubes complicates the overall design because of the additional collimating and shielding that is necessary to prevent x-rays from two separate tubes from illuminating the same linear sensor.
From the foregoing, although several different methods of implementing an x-ray inspection system exist, with each exhibiting its own level of complexity, cost, speed and image quality, a need still exists for an x-ray inspection system that provides accurate, detailed images of the various layers of an article under inspection while significantly reducing overall inspection time and system cost.